*: introduce and use FAST_FUNC: regparm on i386, otherwise no-on

text    data     bss     dec     hex filename
 808035     611    6868  815514   c719a busybox_old
 804472     611    6868  811951   c63af busybox_unstripped

archival/bbunzip.c

@@ -28,7 +28,7 @@ int open_to_or_warn(int to_fd, const char *filename, int flags, int mode)
 	return 0;
 }
 
-int bbunpack(char **argv,
+int FAST_FUNC bbunpack(char **argv,
 	char* (*make_new_name)(char *filename),
 	USE_DESKTOP(long long) int (*unpacker)(void)
 )